1. Field of the Invention
The present invention relates to a method for fabricating metal bumps onto an electronic device such as an LSI wafer, a chip scale package (CSP), a ball grid array (BGA), and a tape carrier package (TCP), and a plate used for fabricating the metal bumps.
2. Description of the Related Art
Recently, high-density mounting of semiconductor devices has been developed for the purpose of miniaturization and weight-reduction of electronic devices including ICs and LSIs, so that the electronic devices have several hundred input/output terminals (electrodes). In a known method for mounting an electronic device having many terminals to a printed circuit board, metal bumps are formed onto a surface of a semicounductor chip (or semicounductor package), and the semicounductor chip is then attached to the printed circuit board via the metal bumps.
A method for mounting metal bumps onto a CSP or a BGA is disclosed in the U.S. Pat. Ser. No. 08/516,284 and Japanese Unexamined Patent Publication (Kokai) No. 7-249631 (entitled a method for fabricating solder bumps and solder balls and a method for fabricating a semiconductor device). This publication discloses a method for forming solder bumps comprising the steps of forming cavities in a solder ball forming plate, filling the cavities with solder paste, and heating the solder ball forming plate to form solder balls in the cavities. After the solder ball forming plate is arranged relative to an electronic device so that the solder balls are aligned with the electrode pads of the electronic device, a reflow step is carried out to transfer the solder balls from the solder ball forming plate to the electrode pads of the electronic device.
FIGS. 10 and 11 of the attached drawings illustrate the solder ball forming plate 10A which is similar to the solder ball forming plate described in the above described publication. The solder ball forming plate 10A has a flat surface and a plurality of cavities 11A (only one is shown in the drawings) arranged in the flat surface thereof. The solder ball forming plate 10A is made from a silicon wafer such that the flat surface of the solder ball forming plate 10A coincides with a &lt;100&gt; crystallographic plane of the silicon. The cavities 11A are formed in the flat surface, by anisotropic etching, using a mask having square openings and the cavities 11A have the shape of a square pyramid. Solder paste is filled in the cavities 11A, using a squeegee, and the solder ball forming plate 10A is heated to form solder balls 2 in the cavities 11A. The solder ball 2 is shown in FIG. 11. After the solder ball forming plate 10A is arranged relative to the electronic device 50 so that the solder balls 2 are aligned with the electrode pads 51, a reflow step is carried out to transfer the solder balls 2 from the solder ball forming plate 10A to the electrode pads 51 of the electronic device 50. The solder balls 2 get wet to the electrode pads 51 to become solder bumps on the electronic device 50.
Each of the solder balls 2 formed in this manner has a diameter "d" which depends on the volume of solder paste in each cavity 11A, and all the solder balls 2 have a generally identical diameter "d". The diameter "d" is such that the solder ball 2 projects upward a distance from the flat surface (i.e., the &lt;100&gt; crystallographic plane) of the solder ball forming plate 10A, the projecting distance being approximately 10 percent (d/10) of the diameter "d". The solder ball 2 can be transferred to the electrode pad 51 since the solder ball 2 projects upward from the flat surface of the solder ball forming plate 10A. However, the projecting distance is relatively small, so there is a possibility that some of the solder balls 2 may not be transferred to the electrode pads 51.
FIGS. 12 to 14 of the attached drawings illustrate another solder ball forming plate 10B, having a plurality of cavities 11B (only one is shown in the drawings), which is described in the U.S. patent application Ser. No. 08/547,532 (U.S. Pat. No. 5,643,831). The solder ball forming plate 10B is also made from a silicon wafer such that the flat surface of the solder ball forming plate 10A coincides with a &lt;110&gt; crystallographic plane of the silicon. The cavities 11B are formed in the flat surface, by anisotropic etching, using a mask having rhombic openings. By this etching, each of the cavities 11B is formed in the shape of a ship bottom. That is, each of the cavities 11B has two oblique bottom surfaces 11E and 11F and four side surfaces extending perpendicular to the &lt;110&gt; crystallographic plane, with the oblique bottom surfaces 11E and 11F defining a deepest portion 11G therebetween. The angle .alpha. between two adjacent sides of the rhombic opening is 70.53 degrees, and the angle .beta. is 54.74 degrees.
Solder bumps can be fabricated in a manner similar to the previous one. That is, solder paste is filled in the cavities 11B, the solder ball forming plate 10B is heated to form solder balls 2 in the cavities 11A, the solder ball forming plate 10B is arranged relative to the electronic device so that the solder balls 2 are aligned with the electrode pads, and the solder balls 2 are transferred from the solder ball forming plate 10B to the electrode pads of the electronic device, by reflowing. The solder balls 2 become solder bumps on the electronic device.
When the angle .alpha. is 70.53 degrees, the depth of the deepest portion 11G is half the length of a shorter diagonal line of the rhombus. Each of the solder balls 2 formed in this manner has a diameter "d" and projects upward a distance from the flat surface (i.e., the &lt;110&gt; crystallographic plane) of the solder ball forming plate 10B, the projecting distance being approximately 20 percent (2d/10) of the diameter "d". Therefore, all the solder balls 2 may be successfully transferred to the electrode pads of the electronic device.
In this case, however, the deepest portion 11G extends parallel to the &lt;110&gt; crystallographic plane, and has a length "l" corresponding to the length of a shorter diagonal line of the rhombus. The length "l" of the deepest portion 11G is longer than the diameter "d" of the solder balls 2, and therefore, the solder balls 2 can move in the respective cavities 11B on the deepest portion 11G. There is a variation of the position of the solder balls 2 in the respective cavities 11B.
In the future, it is expected that, in the ULSIs (ultra LSI), the distance between the adjacent metal bumps become approximately 150 .mu.m and the number of the metal bumps will reach several thousand. In such a case, the metal balls (solder balls) formed in the cavities 11A or 11B should have the diameter in the range from 80 to 90 .mu.m. Therefore, in the case of the solder ball forming plate 10A having the cavities 11A made so that the flat surface coincides with the &lt;100&gt; crystallographic plane, the average projecting distance of the solder balls 2 from the flat surface becomes smaller than 10 .mu.m (10 percent of 80 to 90 .mu.m). If solder paste is not sufficiently filled in the cavities 11A, the projecting distance of the solder balls 2 will be less than this value. Therefore, it is difficult to successfully transfer all the solder balls 2 from the cavities 11A formed in the &lt;100&gt; crystallographic plane to the electrode pads of the electronic device.
On the other hand, in the case of the solder ball forming plate 10B having the cavities 11B made so that the flat surface coincides with the &lt;110&gt; crystallographic plane, the projecting distance of the solder balls 2 from the flat surface is approximately 20 .mu.m. Therefore, regarding the projecting distance of the solder balls 2 from the flat surface, it is easy to successfully transfer all the solder balls 2 from the cavities 11B to the electrode pads of the electronic device. However, the position of the solder balls 2 is not stable in the respective cavities 11B, as described above, so it is difficult to successfully transfer all of several thousand solder balls 2 in the cavities 11B formed by the &lt;110&gt; crystallographic plane to the electrode pads of the electronic device.
In particular, in order to transfer the metal bumps to the electrode pads of the ULSI in which the distance between the adjacent metal bumps is approximately 150 .mu.m and the number of the metal bumps is several thousand without failure, it is important that not only the projecting distance of the metal balls from the flat surface of the solder ball forming plate is relatively greater but also the metal balls are uniformly located in the respective cavities.